This nonprovisional application claims priority under 35 U.S.C. § 119 (a) on patent application Ser. No. 2002-269781 filed in Japan on Sep. 17, 2002, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a method of producing films, in particular, cellulose acylate films suitable for purposes such as a protective film for a sheet polarizer of a liquid crystal display (LCD), an optical compensating film, a color film, and a photographic material.
2. Description of the Related Art
Cellulose acylate films, in particular, a cellulose triacetate film (TAC film) has been used for purposes such as a protective film for a sheet polarizer of a liquid crystal display, an optical compensating film and a color film for the markets which have been growing in recent years, since it is excellent in optical isotropy.
Generally, cellulose acylate films are produced by solvent casting method; specifically, they are produced in the steps of: dissolving a polymer (a macromolecular material) in a solvent (mainly an organic solvent) to prepare a dope; casting the dope on a casting support such as a casting band or a casting drum; stripping the cast dope from the casting support to form a film; and drying the formed film. In the solvent casting method, however, small wrinkles can sometimes occur on the surface of a formed film due to the shrinkage of the film etc., and the occurrence of such wrinkles has been improved generally by conducting tentering, as a wrinkle preventive measure, for the film having been stripped from the casting support so that the film is stretched or regulated in its width direction.
When stripping the formed film from the casting support, it is difficult to apply stripping loads uniformly in the film's width direction, and the distribution and the variation in stripping loads are likely to cause non-uniformity in the stripping force. The variation in stripping force, which acts on the film, from part to part in turn creates a variation in the degree of stretching from part to part of the film, and this variation can sometimes cause a very small non-uniformity in thickness (hereinafter referred to as “cast non-uniformity”) of the film having been stripped. This cast non-uniformity causes deterioration in smoothness and flatness of the film as a product, just as does the aforementioned wrinkles. For the cast non-uniformity, when the film thickness was large, its impact on the film's quality used not to be noticeable; however, in these days, films has become extremely thin, and even very small cast non-uniformity become tangible as deterioration in smoothness and flatness and its impact on the film's quality is large.
The causes of the distribution and variation in stripping loads, which are the factors causing cast non-uniformity, possibly have to do with deposits of the dope etc. depositing and accumulating on part of the casting support surface, non-uniformity in the drying state of the film on the casting support, non-uniformity in the temperature of the casting support itself, and non-uniformity in the concentration of the dope during the casting operation. Moreover, not only the above-described distribution and variation in stripping loads, but also non-uniformity in thickness of the casting film during the casting operation can sometimes remain as cast non-uniformity in the film having been stripped. The occurrence of cast non-uniformity causes the deterioration in appearance of the film, and in addition, it becomes tangible when a coating solution is applied to the film or when the film is laminated to a liquid crystal etc., causing product failures. In particular the films utilized for the optical purpose (e.g., for LCD), whose quality has been significantly enhanced in recent years, cast non-uniformity can be a fatal defect.
As a cast non-uniformity preventive measure, it is essential to improve the solvent casting method to eliminate the distribution and variation in stripping loads, which are the factors causing cast non-uniformity, or to eliminate the non-uniformity in thickness of the casting film during the casting operation. In actuality, however, these improvements also have a limitation. Thus, when the effectiveness of the improvements are not sufficient, the film production rate is decreased, as the ultimate measure, to increase the self-supporting properties of the dope on the casting support and then the dope is stripped from the casting support to form a film, so that casting non-uniformity is hard to produce. This, however, gives rise to another problem of decreasing the productivity.
There have been proposed techniques for improving the smoothness and the flatness of the film produced by the solvent casting method. For example, Japanese Patent Application Publication No. 8-142209 discloses a method of improving the flatness of thermoplastics films which includes: a roller heating step of conveying the thermoplastics film while heating with a far-infrared heater and a heating roller to flatten the film; and a roller cooling step subsequently after the roller heating step. This method, however, has the disadvantage of requiring the reconstruction of equipment for carrying out the solvent casting method. Since the method requires a roller heating step and a roller cooling step in the downstream of the solvent casting method, it cannot employ the tentering step and the roll drying step of the conventional solvent casting method without any modifications.
Japanese Patent Application Publication No. 4-286611 discloses a method of improving linear burrs (continuous wrinkles occurring in the film conveying direction during the conveying operation) on the surface of a cellulose triacetate film in which the cellulose triacetate film is conveyed in such a manner that it engages with rolls while keeping the amount of the solvent remaining in the film 10% or less in first and second drying chambers, keeping the surface temperature of the film in the same chambers lower than that of the film in the last chamber by 15° C. or more, and keeping the surface temperature of the film in the last chamber within the range of its glass transition temperature to the glass transition temperature +40° C. (i.e., 40° C. above the glass transition temperature). This method is to perform high-temperature and high-speed drying while maintaining the flatness of the film, but not eliminate the cast non-uniformity occurring in the film.
Japanese Patent Application Publication No. 11-48271 discloses a method to dry a web stripped from a casting surface while stretching the same with a tentering machine in the environment that meets the requirement: the solvent content in the film of less than 50% by weight and 12% by weight or more. This method has the effect of eliminating the wrinkles having occurred on the film surface and the cast non-uniformity having occurred in the film conveying direction; however, since the cast non-uniformity is formed in a variety of directions, including the film conveying direction and film width direction, the cast non-uniformity formed in the directions other than the film conveying direction is rather fixed by the tentering and thereby is difficult to rectify in a later step.
Thus, in actuality, if the prior art, which is to improve the smoothness and the flatness of films produced by the solvent casting method, is applied to the improvement of cast non-uniformity without any modifications, sufficient effect cannot be produced.